Fluid pressure system having open center and closed center portions



April 4, 1961 w 2,977,761

R. FLUID PRESSURE SYSTEM HAV OPEN CENTER AND CLOSED CENTER TIONS Filed Oct. 51, 1957 3 Sheets-Sheet 1 C) (D) @D INVENTOR.

flaw/20 z. ZfW/S ,4 TTORMEYJ April 4, 1961 R. L. LEWIS 2,977,761

FLUID PRESSURE SYSTEM HAVING OPEN CENTER AND CLOSED CENTER PORTIONS Filed Oct. 31, 1957 3 Sheets-Sheet 2 INVENTOR.

. flc'l /ma A. ZfW/J April 4, 1961 R L w s 2,977,761

FLUID PRESS S YS HAVING OPEN CENTER AND SED TER PORTIONS Filed Oct. 51, 1957 3 Sheets-Sheet 5 INVENTOR.

Mew/41m Z A W/5 BY FLUID PRESSURE SYSTEM HAVING OPEN CEN- TER AND CLOSED CENTER PORTIONS Richard L. Lewis, St. Joseph, Mich., assignor to The Bendix Corporation, a corporation of Delaware Filed Oct. 31, 1957, Ser. No. 693,674

s Claims. or. 60-51) The present invention relates to a fluid pressure system having an open center portion, an accumulator, and means for charging the accumulator from the open center portion of the system; and 'more particularly to a hydraulic system having an accumulator charged from an open center system by means which only divert a generally fixed amount of the flow through the open center portion of the system to the accumulator.

An object-of the invention is' the provision of a new and improved fluid pressurecircuit having an open center portion through which fluid is continually circulated, an accumulator adapted to be maintained at a generally predetermined high-pressure level, and means which will not completely close oif the flow through the open center portion of the system at any time but which will cause the accumulator to be charged from the open center portion of the system at a generally predetermined fixed rate.

Another object of the invention is the provision of a new and improved system of the above described type having back pressure valve'rneans' in the open center circuit for producing a pressure in the open center circuit which is maintained during the charging operation of the accumulator at a pressure which exceeds the pressure in the accumulator by a generally fixed amount.

Another object of the invention is the provision of a new and improved system of the above described type having means therein which causes the back pressure valve to be brought into operation when the pressure level in the accumulator drops below a generally predetermined level, and which deactivates the back-pressure valve when the pressure in the accumulator reaches a higher generally predetermined level.

A further object of the invention is the provision of a new and improved system of the above described type wherein the back-pressure valve is actuated by means of a fluid pressure motor having a pair of opposing chambers, one of which expands to open the back-pressure valve, and the other of which expands to cause the valve to produce its back-pressure; and in which the back-pressure created is conducted to the first mentioned opposing chamber, and pressure from the accumulator is communicated to the other opposing chamber to cause a backpressure to be created in the open center. system which exceeds that in the accumulator by a generally fixed pressure differential.

The invention resides in certain constructions, and combinations, and arrangements of parts; and further objects and advantages will become apparent to those skilled in the art to which the invention relates from the following description of the preferred embodiment described with reference to the accompanying drawings forming apart of this specification, and in which:

Figure 1 is a side elevational view of a valve which embodies principlesof the present invention; Figure 2 is a right end elevational view of the valve shown in Figure l;

Figure 3 is a cross sectional view taken approximately on 'th'e line 3--3 of Figure '2; and in which, components atent O of a combined power braking and steering system of an automotive vehicle are schematically shown to illustrate the improved fluid pressure system;

Figure 4 is a cross-sectional view taken approximately in the plane indicated by section lines -44 in Figures 1 and 3;

Figure 5 is a cross-sectional view taken approximately in the plane indicated by the line 5-5 in Figures 1 and 3.

Although the invention may be embodied in other types of fluid pressure circuits, it is herein shown and described as embodied in a hydraulic circuit for power actuating the brake and steering systems of an automotive vehicle. The system shown in Figure 3 of the drawing generally comprises a positive displacement pump A which is supplied with hydraulic fluid from a reservoir B, and which continually circulates hydraulic fluid through a valve C (about to be described) to a power steering control valve D. The power steering control valve D is of the open center type which passes the flow therethrough without producing any appreciable back-pressure in the system when the vehicles steering gear is'not being actuated; and which produces just suflicient' back-pressure against the pump A, when the control valve D is actuated, to move the steering gear of the vehicle. Under all normal conditions flow passes through the control valve D back to the reservoir B, even when the control valve D is actuated, except in those instances where the actuation of the steering gear requires a pressure or an amount of fluid which is greater than can be supplied by pump A.

The system shown in Figure 3 also include an accumulator E adapted to be maintained at a generally predetermined elevated pressure level, and a vehicle brake P which is adapted to be actuated by the pressure supply maintained in the accumulator E. Operation of the vehicles brakes is controlled by a valve portion C1 in the housing C which regulates the amount of pressure that is supplied to the vehicle brake F from the accumulator E. A valve portion C2 is provided in the housing -C for developing sufiicient back pressure against the pump A to periodically charge the accumulator E. There is also included in the valve housing C a pilot valve portion C3 which actuates the charging valve portion C2 to cause the accumulator E to be charged whenever the pressure in the accumulator falls below a generally predetermined lower level; and which deactivates the charging valve portion C2 when the pressure in the accumulator E reaches a higher generally predetermined pressure level. The valves C1, C2,

and C3 may, of course, be formed in individual body members suitably interconnected in the system; but in the preferred arrangement, will be formed within a single integrally cast housing member.

The control valve portion C1 .for operating the brakes of the vehicle isformed by means of a longitudinally extending boring 10 extending through the valve housing C. The boring 10 is counterbored, as at 12, and the end spaced from the operator (left-hand side as seen in Figures 1 and 3) is closed off by .a threaded plug 14. An annular flanged seat member 16 is positioned in the longitudinally extending bore 10 with its radially outwardly extending flange 18 in abutment with the shoulder 20 provided by the counterbore 12 to form a rear valve chamber 22 adapted to be communicated to the accumulator 'E bymeans of a threaded opening or accumulator port 24 (Figure 1) extending through the side wall of the housing. The portion of the longitudinally extending bore 10 adjacent or closest to the operator is adapted to be communicated with the reservoir B by means of a threaded opening or exhaust port .26 in the sidewalls of the housing C; and the portion of the longitudinally extending bore 14) intermediate the accumulator and exhaust ports 24 and .26, respectively, is adapted to be communicated -with the vehicle brake F Patented Apr. 4, 1961 by means of another threaded opening or control port 28 in the sidewall of the housing C. An O-ring seal 30 is provided between the sidewalls of the bore and the annular valve seat member 16; and flow communication between the accumulator port 24 and the exhaust port 26 is controlled by means of one end portion of a dumbbell shaped valve closure member 32 which abuts the rear side of the valve seat member 16 to close off the valve port or central opening 34 through the valve seat member. The dumbbell shaped valve closure member 32 extends through the valve port 34 in such manner that the other enlarged end of the dumbbell shaped valve closure member is positioned forwardly of the valve seat member 16; and flow communication between the control port 28 and the exhaust port 26 is controlled by means of a movable control member 36 which is adapted to abut the front end of the dumbbell shaped valve closure member. The movable control member 36 is a generally spool shaped member whose land portions straddle the exhaust port 26, such that its annular recess between the lands is always in communication with the exhaust port 26. Flow communication between the control port 28 and the exhaust port 26 is provided by means of a longitudinal drilling 38 in the rear end of the control member 36, and

- a transverse drilling 40 which communicates the drilling 38 with the annular recess 42 in the control member 36. The rear face of the control member 36 is bevelled to provide a valve seat 44 on the control member 36 adapted to abut the front face of the dumbbell shaped valve closure member 32 and close ofi the longitudinal drilling 38. Suitable seals are provided on each of the land portions of the spool shaped valve control member 36; and the front end of the control member 36 is bored out to receive a rubber block 46 and a washer 47 against a pushrod 48 abuts for actuating the brake control valve portion C1. In order that the pressure forces upon the dumbbell shaped" valve closure member 32 might be essentially balanced, the valve seats in the annular member 16 and the control member 36 are made to be of approximately the same diameter; and a slidable tubularly shaped sleeve 50 is positioned against the rear face of the dumbbell shaped valve closure member 32 to seal off the accumulator pressure from the rear face of the closure member from an area on the rear face of the closure member approximately equaling that of the valve seats in the member 16 and the control member 36. The dumbbell shaped valve closure member 32 is provided with a longitudinal opening 52 therethrough to communicate the area within the slidable tubularly shaped sleeve 50 with the longitudinal drilling 38 of the valve closure member 36; such that the same pressure exists in both the front and rear faces of the closure member 32. The tubularly shaped sleeve 50 is slidingly sealingly received in a bore 54 in the rear end plug 14, and a coil spring 56 is provided to at all times hold the sleeve 50 in sealing engagement with the rear end of the dumbbell shaped valve closure member 32.

The brakes of the vehicle are operated by depressing the pushrod 48 into the longitudinally extending bore 10 causing the rear face of the control member 36 to firmly abut the front face of the valve closure member 32 to prevent communication between the vehicle brake F and the exhaust port 26, and thereafter bias the valve closure member 32 rearwardly to move its rear portion out of engagement with the valve seat member 16, and thereby permit pressure from the accumulator E to flow to the vehicle brake F. The tubularly shaped sleeve 50 will, of course, be held into engagement with the rear face of the valve closure member 36 during inward movement of the valve closure member by means of the coil spring 56; and any leakage that should reach the inside of the tubularly shaped sleeve 50 will, of course, be conducted to the cumulator E into the braking system, control pressure against the rear face of the control member 36 will substantially equal the force being applied to the pushrod 48 by the operator, and the control member will be biased outwardly against the foot of the operator a sufiicient distance to permit the valve closure member 32 to again abut the valve seat member 16. Further rise in pressure in the brake system is thereby prevented, and a generally fixed braking effort will be maintained. Upon the reduction of force upon the pushrod 48 by the operator, the control pressure on the inner end of the control member 36 will bias the control member out of engagement with the valve closure member 32 to permit pressure to escape through the drilling 38 in the closure member 36 to the exhaust port 26. When the pressure in the braking system reduces to a point where the force exerted on the inner end of the control member 36 just equals the force being maintained by the operator upon the pushrod 48', the cotnrol exhaust port 26 through the longitudinally extending member 36 will again abut the closure member 32 and prevent a further reduction in pressure in the braking system. In the preferred arrangement a spring, not shown, will be used to bias the pushrod 48 inwardly with a predetermined small amount of force to provide a small residual pressure in the braking system when the foot of the operator is removed from the pushrod 48.

As previously indicated, the power steering valve D is an open center valve such that fluid is continually discharged from the pump through the valve C to the control valve D and back to the reservoir B at a pressure which is considerably less than that desired to be maintained within the accumulator E. It is a function of the charging valve portion C2 to build up sufficient back pressure against the pump A to permit flow to proceed from the charging valve C2 to the accumulator E. The charging valve portion C2 is formed by means of a longitudinally extending boring 60 extending through the valve housing C generally parallel with respect to the control valve C1. The rear end of the boring 60 is closed off by a threaded plug 62, and the front end of the boring 60 is closed off by an adjustable threaded plug 64. A pair of annular recesses 66 and 68 are provided in the sidewalls of the longitudinally extending boring 60the rear annular recess 66 of which is comunicated with the power steering valve D by means of a fiow through port 70 in the valve housing C, and the interconnecting line 72; and the front annular' recess 68 of which is communicated with the pump A by means of a pump pressure port 74 in the housing C, and the interconnecting line 76.

.A spool shaped valve member 78 is positioned in the longitudinally extending boring 60 with its opposite end land portions 80 and 82 positioned on opposite sides of the annular recesses 66 and 68. The land portions 80 and 82 are in substantial sliding sealing engagement with respect to the sidewalls of the longitudinally extending bore 60 to form opposite end opposing fluid pressure chambers 84 and 86 which expand and contract alternately when the spool member is reciprocated within the boring 60. A central boss or projection 88 is formed on the adjustable threaded plug 64 for abutment with the front face of the spool shaped valve member 78 to limit forward movement of the spool shaped valve member 78. When the valve member 78 is in abutment with the adjustable projection 88, the rear edge of the front land portion 82 of the spool 78 will be positioned forwardly of the portion 90 of the sidewalls of the longitudinally extending boring 60 which separates the annular recesses 66 and 68. When the spool shaped valve member 78 is so positioned, flow from the pump A passes through the pressure port 74 into the recess 68, through the clearance provided between the land 82 and the sidewall portion 90, to the rear recess 66, fiow through port 70, interconnecting lines 72, and hence to the reservoir B through the power steering control valve D. 1

Under such conditions, only sufficient pressure is develin the accumulator E, flow proceeds from the recess .68

through a horizontal drilled passageway .92 to ,a vertical drilling 94 that communicates with-the counterbore 12, of the brake control valve C1, which communicates with the accumulator E. The outer end of the vertical drilling 94 is closed ,oif .by a ball plug 95, andtheouter end of the horizontal drilling 92 is closed off by a threaded plug ,96. A valve seat member 98 is pressed into' the portion ,of the horizontal drilling 92 between the boring 6t) and .the vertical drilling 94 to provide a bevelled seat 188 for a a .ball check .valve 102 which prevents flow from proceeding from the accumulator back into the charging valve C2. The ball 102 is biased against the seat 100 by a coil spring 104 which isreceived'wit-hin a bore 106 in the inner end of the plug 96. Thefiow conducting port 108 through the annular 'valve seat member 98 is sized to provideja generally predetermined pressure drop thereacross for a given rate of flow. In the present instance. the port 108 Will provide a pressure drop of approximately 35 pounds for a rate of fluid flow of approximately 1 gallon per (minute.

According to principles of the present invention, the charging valve :02 is so constructed and arranged that it normally does not completely close olf flow from the pump -A to the power steering valve D when it is charging the accumulator E; and that it will only produce a back .pressure in the recess 68 greater than that in the accumulator E by the pressure drop at which the flow conducting port 108 provides the desired rate of flow to the accumulator E. In the present instance the charging valve C2 produces a pressure during the accumulator charging operation which exceeds that in the accumulator E by approximately 35 pounds per square inch pressure.

In order to limit the amount of back pressure developed by the charging valve C2, the 'back pressure created within the recess 68 is communicated directly to the rear end of the spool shaped valve member 78 by means of a a transverse drilling 110 and a longitudinal drilling 112 in the spool member 78. With such an arrangement, the back pressure developed by the valve produces a force upon the spool member 78 tending to reduce the amount of back pressure being created by the charging valve. Pressure at substantially accumulator level will be supplied to the opposing fluid pressure motor chamber 86 during theaccumulator charging operation by means of a drilling 114 (Figure 4) which communicates the horizontal drilling 92 on the accumulator :side of flow restriction 108 'to the pilot valve portion -C3--which during the accumulator charging operation communicates the pressure to a diagonal drilling 116 that communicates with the fluid pressure motor chamber 86. With such an arrangement, the valve opening pressure in the motor chamber 84 will exceed the valve closing pressure in the motor chamber 86 by approximately 30 pounds per square inch pressure; and in the preferred embodiment, a coil spring 118 is provided in the motor chamber 86 to provide a valve closing force of approximately 30 .p.s.i., such that the valve will become balanced when the backpre'ssure in the recess 68 exceeds that in the accumulator E by the amount of 35 psi. pressure.

It is desiredthat the charging operation for the accumulator B will :begin at some generally predetermined level,

and will "continue until the pressure in the accumulator reaches "some generally predetermined higher level. The charging operation of the charging valve portion C2 is controlled by means of the pilot valve portion C3which the present instance "supplies accumulator pressure from the downstream side of the flow restriction 108 to the .motor chamber 8.6 when the accumulator pressure falls to approximately 1,175 p.s.i., and which relieves the pressure Within the motor chamber 86 when the pressure in the accumulator reaches approximately 1,400 psi. The pilot valve C3, which controls the operation of the charging valve C2, is formed by means of a longitudinal bore through the valve housing C extending generally parallel with the valve portions C1 and C2. The rear end of the bore 120 is counterbored as at 122 to provide a valve chamber into which the accumulator pressure is bled by the passageway 114. The forward end of-the longitudinal bore 120 is also counterbored as at 124 .to provide an enlarged valve chamber which communicates with the longitudinalbore 10 of the valve C1 in such manner as to always communicate with the exhaust port 26 through the passage 126. The rear end of the bore 120 is closed off by a threaded plug 128 and the front end of the bore 120 is closed off by a threaded adjustable plug 130 whose adjustable function will later be described.

The pilot valve C3 is completed by a flanged sleeve member 132 which is pressed into the bore 120 with its flange in abutment ,with the shoulder 134 provided by means of the 'counterbore 122. The internal passageway 136 through the sleeve member 132 forms a valve port which communicates thefront and rear chambers 124 and 122 of the valve, and the opposite ends of the internal passageway 136 are bevelled or chamfered to provide seating surfaces 138 and 140 for a pair of identically shaped spherical ball valve closure members 142. The outer surface of the sleeve member 132 is recessed as at 144 to form an annular space into which the diagonal drilling 116 communicates, and a radial drilling 146 causes the internal passage 136 of the pilot valve to at all times be communicated with the opposing fluid pressure motor chamber 86 of the charging 'valve C2.

It will be seen that pressure from the accumulator to the chamber 86 is controlled by the rear ball 142, and that .pressure between the chamber 86 of the valve C2 and the exhaust chamber 124 is controlled by the forward ball "142. In order that pressure from the accumulator will be respect to the internal passage 136 is positioned between the balls 142. The pin 152 is provided with a predetermined length which will permit only one of the balls to be in abutment with its seat at any one time. When the front ball 142, therefore, is biased .into engagement with its seat by the coil spring 148, the rear ball 142 will be lifted from its valve seat 138 to conduct accumulator pressure through the space between the pin 152 and internal passage 136 to the radial drilling 146, and thereby pressurize the motor chamber 86 of the charging valve C2 to start the charging operation of the accumulator. When the accumulator has been charged to some upper limit (the present instance 1,400 p.s.i.), suflicient pressure will exist against the inner end of the ball 142 to lift it off of its seat 140 againstthe force being applied by the spring 148. As previously explained, the pin 152 is of just sufficient length so that when the front ball 142 moves out of engagement with its'seat when the rear ball142 moves into engagement with its seat 138. Thereafter, accumulator pressure is valved off from the" motor chamber 86 of the charging valve'C2, and at the same time is communicated past the front ball 142 to the exhaust chamber 124 to relieve the pressure within the motor chamber 86 of the charging valve C2. This, of course, "stops furthercha'rging of the accumulator E.

4 In ordertha t a spread in pressure, or a pressure differential, will be provided between the time at which the charging of the accumulator stops and the charging of the accumulator begins, the valve seats 138 and 140 are made ofdifferent diameters by the expediency of chamfering the seats at different angles. In the present instance, the rear seat 138 is chamfered at an angle of 60, while the front seat 140 is chamfered at an angle of 75. Inasmuch as the same size ball is used for a closure member for both seats, it will be seen that the valve seat area which controls communication between the motor chamber 86 and the accumulator E will have a larger area than the seat 148 which controls communication of the motor chamber 86 and the exhaust chamber 124. Once the front ball 142 has been moved olf of its seat and the rear ball 142 has been moved into engagement with its seat 138, the accumulator pressure working on the increased diameter provided by the seat 138 will sufficiently overpower the spring 148 to permit the pressure in the accumulator to drop to its lower level before the force of the spring 148 will again equal the ressure forces opposing the spring to permit the rear ball 142 to be moved out of engagement with its seat 138. Once the rear ball 142 moves out of engagement with its seat, the pressure force against the spring is considerably reduced; such that the front ball 142 will slam against its seat 140 to thereafter communicate the accumulator pressure to the motor chamber 86 of the charging valves C2. As previously explained, this will take place in the present instance at a pressure of approximately 1,175 p.s.i.

In the normal operation of the combined power steering and braking system shown in the drawing, the discharge from the positive displacement pump A will pass through the line 76 at a pressure of approximately 40 p.s.i., through the charging valve C2 which will normally be in its de-energized condition, to the open center power steering valve D, and thence to the reservoir B. When the steering system of the vehicle is actuated, power steering valve D will partially close to produce a back pressure in the line 72 which will be conducted to the power cylinder of the power steering device to produce the necessary force to operate the steering gear of the vehicle. In all but the most extreme conditions the back pressure developed by the steering valve D will not approach 1000 p.s.i. As previously explained, it is desired to control the pressure in the accumulator E, between a lower limit of 1175 p.s.i. and an upper limit of approximately 1400 p.s.i. The control of the braking system is accomplished by the valve portion C1 whose construction has previously been described in considerable detail so that its operation will not now be elaborated upon.

As previously explained, the operation of the charging valve C2 through which the discharge of the pump A continually circulates is controlled by the pilot valve C3. Once the accumulator E has been charged to its upper limit of 1400 p.s.i. pressure the rear ball 142 of the pilot valve will be in engagement with its seat 138, and the front ball 142 will be out of engagement with its seat 140; so that the actuating motor chamber 86 of the charging valve C2 will be in its de-energized condition shown in the drawing.

In the normal condition of the valve C2, substantially no pressure exists in the motor chamber 86; while the normal back pressure of 40 pounds exists in the rear motor chamber 84 of the charging valve C2, by reason of the passages 110 and 112 in its spool shaped valve members 78. As previously explained the spring 118 only exerts a force of approximately 30 pounds; so that, under normal conditions, the spool member 78 is held in engagement with its stop 88 as shown in Figure 3 of the drawing.

Pressure from the accumulator E is continually supplied to the rear chamber 122 of the pilot valve C3 through the counterbore 12 passages 94, 92, and 114, as seen in Figure 4 of the drawing. When the pressure in the accumulator E drops to approximately 1 175 p;s.i., the spring 148 of the pilot valve C3 will exert sufficient force through the front ball 142, and pin 152, to lift the rear ball 142 out of engagement with its seat 138. This considerably reduces the pressure forces exerted against the spring 148, and the front ball 142 will thereafter slam into engagement with its seat 140 to close off communication between the motor chamber 86 of the charging valve C2 and the exhaust chamber 124 of the pilot valve. Pressure from the accumulator E is thereafter bled past the clearance space between the pin 152 and the internal passageway 136 to the motor chamber 86 through the radial drilling 146 and the diagonal drilling 116.

When the pilot valve C3 conducts accumulator pressure to the motor chamber 86 of the charging valve C2, the spool member 78 is immediately forced rearwardly to produce a throttling action between the rear face of its front land portion 82 and the portion 98 of the sidewalls which is positioned between the pump port 74 and the flow through port 70. Pressure immediately builds up in the recess 68, and hence in the opposing motor chamber 84 by reason of the communicating passages and 112, to oppose the actuating forces produced upon the spool member 78 by the accumulator pressure within the motor chamber 86. Inasmuch as the actuating force upon the spool 78 will be the sum of that produced by the accumulator pressure within the chamber 86 and the force of the spring 118, the spool member 78 will be forced in its actuating direction until it produces a pressure in the recess 68, and hence motor chamber 84, which is greater than the accumulator pressure by the amount which will oppose the actuating force of the spring 118. In the present instance the spring 118 is sized to require a pressure to be created in the recess 68 which will be approximately, 35 p.s.i. greater than that delivered to the motor chamber 86 from the accumulator E. Inasmuch as the back pressure produced by the charging valve C2 is now higher than the pressure existing in the accumulator E, flow will proceed from the recess 68 through the flow conducting port 108, past the check valve 102 to the drilling 94, which communicates with the counterbore 12 and the port 24 that is connected to the accumulator.

As previously explained, it is desired that the accumulator E will be charged at substantially a fixed rate (which in the present instance is approximately 1 gallon per minute) such that the flow of fluid to the power steering valve D will not be noticeably reduced during the charging operation of the accumulator. It has also been explained that the flow conducting port 108 is sized to limit the flow rate to the accumulator to the desired rate when the pressure differential across the port 108 created by the charging valve C exceeds the pressure in the accumulator by a fixed amount (in the present instance the back pressure created by the charging valve C2 exceeds that in the accumulator E by a dilferential of approximately 35 p.s.i.). It will, therefore, be seen that as the pressure in the accumulator rises from its lower limit, to its upper limit (in the present instance from 1175 p.s.i. to 1400 p.s.i.), the back pressure created by the charging valve C2 will rise correspondingly; and will at all times exceed that in the accumulator E by the desired pressure differential (which in the present instance is 35 p.s.i.). Flow, therefore, will continue to the accumulator E at a constant rate until the upper limit of 1400 p.s.i. is reached. Thereupon, the force exerted upon the front ball 142 of the pilot valve C3 will just exceed the force exerted thereagainst by the spring 148; so that the front ball 142 will be lifted from its seat 140. When this occurs, the rear ball 142 will be snapped into engagement with its seat 138 to isolate the accumulator E from the actuating motor chamber 86 of the charging valve C2; and thereafter the pressure in the motor chamber 86 is dumped to the exhaust chamber 124 of the pilot valve C3. When this occurs the spool 78 of the charging valve C2 moves to its non-throttling position against the stop 88, and thereby deactivates the charging valve C2. The pressure in the recess 68 will thereupon drop to the back pressure created by th power steering valve D, and the check valve 102.will slam into engagement with its seat 100 to prevent back flow from the accumulator E into the open center pumping circuit.

When the pressure in the accumulator E drops to its lower limit again (in the present instance 1175 p.s.i.), the force of the spring 148 in the pilot valve C3 will again be suflicient to lift the rear ball 142 out of engagement with its seat '138; and the charging valve C2 will again be energized toinitiate the charging cycle of the accumulator. I

It will be apparent to those skilled in the art that other aware I types of actuating control can be used for the charging valve C2; and that the actuating biasing forces upon the charging valve C2 which produces the desired pressure difierential can be created by means other than a spring, as for example'by the use of a land portion 82 having a larger area than the'oppositeend land portion 80. While the invention has been described in considerable detail, I do not wish to be limited to the particularconstructions shown and described, and it is my intention to cover hereby all novel adaptations, modifications, and arrangements thereof which come within the practice of those skilled in the art to which the "invention relates and which come within the puriew of the following claims.

I claim:

1. In a fluidtpressure system for operating one or more fluid pressure actuated devices one of Which is intended to receive a substantially constant flow of fluid; a pump having a suction and a discharge, a back pressure valve and orifice controlled thereby for exerting back pressure against the discharge of said pump, means for communicating a substantially continuous flow of fluid received at the downstream side of said orifice to the pressure actuated device requiring such constant flow, an accumulator, flow conducting means communicating back pressure produced by said back pressure valve to said accumulator and including a pressure-drop-producing resistance to fluid flow to said accumulator, check valve means in said flow conducting means for preventing back flow from said accumulator to said back pressure valve, housing means providing a fluid pressure chamber for the back pressure developed by said back pressure valve and communicating at all times through said orifice with the fluid conducting means leading to said constant-flow-receiving pressure actuated device, a pressure responsive movable wall operatively connected to said back pressure valve and movable in one direction to oppose the throttling action at said orifice effected by said back pressure valve and thereby tending to reduce the back pressure magnitude, means for supplying said movable wall with the back pressure produced by said back pressure valve upstream of its pressure-drop-producing resistance, first means for urging said back pressure valve to a throttling position'and thereby opposing said movable wall, and second means responsive to pressure in said accumulator and actuated by a pressure drop in said accumulator to a generally predetermined value to communicate pressure to said first means which initiates throttling action by said back pressure valve by movement thereof to a balanced throttling position wherein back pressure on said movable wall opposes said first means with an equal force and said fluid pressure chamber is maintained at an increased pressure and in communication with said accumulator and means responsive to accumulator pressure and actuated by said accumulator pressure at a generally predetermined maximum value to deactuate said second means and thereby reduce the back pressure effected by said back pressure valve at its balanced position.

2. In a fluid pressure system for operating one or more fluid pressure actuated devices one of which is intended to receive a substantially constant flow of fluid; a pump having a suction and a discharge, a back pressure valve and orifice controlled thereby for exerting back pressure against the discharge of said pump, means for communicating a substantially continuous flow :off fluid received at the downstream side of said orifice to ,thepressur'e actuated device actuated thereby, an accumulator, flow conducting means communicating back pressure produced by said a back pressure valve to said accumulator and including a pressure-drop-producing resistance to fluid flow to said accumulator, check valve means in said flow conducting means forpreventing back flow from said accumulator to said back pressure-valve, means defining a fluidpressure chamber which contains fluid at a back pressure value determined by said back pressure valve and in constant communication through said orifice with the constantflow pressure-actuated devices, fluid pressure operated motor means for positioning said back pressure valve and including a pair of opposing fluid pressure chambers, one

= of which when expandedmoves said back pressure valve to a pressurerelieving position and the other of which when expanded moves said backpressure valve .to a throttling position to produce a back pressure against said pump, said one opposing chamber beingsupplied with the back pressure developed in said chamber by said backpressure-producing valve and the other of said opposing chambers receiving a pressure from said accumulator, means responsive to accumulator pressure and actuated thereby at a predetermined lower limit of accumulator pressure to regulate communication of accumulator pressure to said other opposing chamber which effects displacement of said back pressure valve to a position where increased throttling produces higher back pressure values suflicient when communicated to said one opposing chamber to balance said back pressure valve in a position maintaining fluid flow through said orifice and fluid flow to said accumulator for charging the same, the pressures supplied said two opposing chambers remaining at a substantially fixed differential amount to maintain a substantially constant rate of charging of said accumulator, and means responsive to pressure within said accumulator and actuated at a predetermined upper accumulator pressure to relieve pressure in said other opposing chamber whereby said back pressure valve moves to a balanced position effecting substantially reduced back pressure accumulator, flow conducting means communicating back pressure produced by said back pressure valve to said accumulator and including a pressure drop producing resistance to fluid flow to said accumulator, check valve means in said flow conducting means for preventing back flow from said accumulator to said back pressure valve, means defining a fluid pressure chamber which contains fluid at the back pressure value determined by said back pressure Valve and in constant communication through said orifice with the constant-flow pressure-actuated device or devices, fluid pressure operated motor means for positioning said back pressure valve and including a pair of opposing fluid pressure chambers, one of which when expanded displaces said back pressure valve to a pressure relieving position and the other of which is communicated with accumulator pressure and when expanded displaces said back pressure valve to a throttling position in relation 'to said orifice to produce a higher back pressure against said pump, spring means for supplementing the force developed by pressure in said other chamber whereby accumulator pressure and back pressure are maintained at a generally predetermined differential.

4. A combined power steering and power braking system comprising a pump, a chamber having a fluid inlet connecting with said pump and an outletleading to the power steering portion of said system to provide a continuous flow of fluid from said pump to said power steering, a pressure accumulator maintained at pressures between two predetermined limits, a second outlet from said chamber having communication means with said accumulator and providing flow only in the direction from said chamber to said accumulator, back pressure providing means for controlling pressure within said chamber, opposed fluid pressure responsive means for operating said back pressure producing means, means for communicating the back pressure developed by said back pressure producing means to one of said opposing fluid pressure responsive means and accumulator pressure to the other of said opposed means whereby pressure within said chamber is maintained at a substantially fixed higher pressure diflerential than accumulator pressure during charging thereof and with said outlet at all times being open to maintain fluid flowfrom the pump through said chamber and outlet to said power steering portion, and pressure sensitive means in communication with said accumulator to etfect communication of accumulator pressure at a lower predetermined value to one of said opposed means for increasing the throttling effect of said back pressure producing means, and means actuated by accumulator pressure at its higher predetermined value to terminate pressure transmitted to said one opposed means and end the charging of said accumulator.

5. A fluid pressure system for operating a plurality of pressure actuated devices comprising a pump, a back pressure producing means connected with said pump, one of said pressure actuated devices being also connected to said back pressure producing means at the downstream portion to receive a substantially continuous fluid flow from said pump, two opposed fluid pressure operated motors for controlling said back pressure producing means to regulate the amount of back pressure produced thereby while maintaining said continuous flow substantially unaffected thereby, one of said fluid motors being operated by upstream pressure produced by said back pressure producing means and the other of said motors being operated by fluid pressure which is maintained at a lower and substantially constant differential value of said back pressure and urging said back-pressure-producing means in a direction to increase said back pressure, an accumulator for operating other pressure actuated devices of said system, means for communicating pump pressure with said accumulator when the pressure developed by said back pressure producing means exceeds accumulator pressure, means responsive to accumulator pressure and operative at a predetermined lower limit of accumulator pressure to communicate increased pressure to said other motor effecting buildup of back pressure for charging said accumulator, and means for relieving pressure communicated to said other motor at a predetermined upper limit of accumulator pressure to reduce the back pressure developed by said back-pressure-producing means and thereby terminate accumulator charging.

6. A fluid pressure system for operating a plurality of fluid pressure actuated devices one at least of said devices receiving a substantially continuous flow of fluid at its required operating pressure and the other of said devices being indirectly operated by accumulator fluid under pressure, said system comprising a pump, controllable back-pressure-producing means acting against said pump and having fluid communication at the downstream end thereof with those of said devices receiving a continuous flow, means for controlling said back-pressure-producing means to regulate the amount of back pressure produced thereby and including two opposed fluid motors one responsive to back pressure and the other responsive to a second pressure which is balanced by said back pressure means at a predetermined magnitude of back pressure and under continual communication at the downstream end with said continuous-flow-receiving devices,

an accumulator having a fluid conducting connection with said back-pressure-producing means and a flow regulating means providing fluidpassage in an accumulator charging direction only, means responsive to accumulator pressure and operative at a predetermined lower pressure therein to effect energization of said other motor and thereby increasing back pressure to a level obtaining balance position of said back-pressure-producing means wherein fluid continues to flow to said devices and back pressure is communicated to said accumulator as charging pressure and means also responsive to accumulator pressure and actuated at a predetermined upper limit thereof to de-energize said other motor and thereby effect a reduction of back pressure required to balance said back-pressure-producing means and terminate charging of said accumulator.

7. A fluid pressure system for operating a plurality of fluid pressure actuated devices one at least of said devices receiving a substantially continuous flow of fluid at its required operating pressure and the other of said devices being indirectly operated by accumulator fluid under pressure, said system comprising a pump, controllable backpressure-producing means acting against said pump and communicating at its downstream end to those of said devices receiving a continuous flow, means for controlling said back-pressure-producing means to regulate the amount of back pressure produced thereby and including two opposed fluid motors one responsive to back pressure developed by said back-pressure-producing means and the other responsive to a second pressure suflicient to balance its opposed motor at an equilibrium position of said back-pressure-producing means having a continual communication at the downstream end with said continuousflow-receiving devices, an accumulator having a fluid connection with said back-pressure-producing means, means restricting rate of fluid flow from said back-pressure-producing means to said accumulator to an amount less than that receivable by said continuous flow receiving devices and when back pressure exceeds accumulator pressure for accumulator charging, means responsive to accumulator pressure and operative at a predetermined lower pressure therein to effect energization of said other motor at a pressure which is proportional to accumulator pressure and thereby increasing back pressure to a level obtaining balance position of said back-pressure-producing means wherein fluid continues to flow to said continuous-flowreceiving devices and balance pressure is communicated to said accumulator as charging pressure, spring means for supplementing the biasing effect of said other motor and means also responsive to accumulator pressure and actuated at a predetermined upper limit thereof to de energize said other motor and thereby elfect a reduction of back pressure required to balance said back-pressure producing means and terminate charging of said accumulator.

8. A fluid pressure system for operating a plurality of fluid pressure actuated devices one at least of said devices receiving a substantially continuous flow of fluid at its required operating pressure and the other of said devices being indirectly operated by accumulator fluid under pressure, said system comprising a pump, controllable back-pressure-producing means acting against said pump and connected at its downstream end to those of said devices receiving a continuous flow, means for controlling said back-pressure-producing means to regulate the amount of back pressure produced thereby and including two opposed fluid motors one responsive to the back pressure and the other responsive to a second pressure providing balancing of said back pressure-producing means at a predetermined magnitude of back pressure and with a continual communication at its downstream end with said continuous-flow-receiving devices, an accumulator having a fluid conducting connection with said backpressure-producing means to receive fluid pressure charge when back pressure exceeds accumulator pressure, flow regulating means providing fluid passage from said backcontinues to flow to said devices and the balancing backpressure is communicated to said accumulator as charging pressure, means also responsive to accumulator pressure and actuated at a predetermined upper limit thereof to de-energize said other motor and thereby eifecting a reduction of back pressure required to balance said backpressure-producing means to terminate charging of said accumulator and control means for communicating accumulator pressure to other of said devices and independently of said fluid operated devices receiving a substan- 5 tially continuous flow.

References Cited in, the file of this patent UNITED STATES PATENTS Mercier Sept. 25, 1956 2,813,536 Gordon Nov. 19, 1957 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent N0o 2 977fl61 April 4,, 1961 Richard Lo Lewis It is hereby certified that error appears in the above numbered pat- 1 ant requiring correction and that the said Letters Patent shouldread as corrected below.

Column 3 line 34 after "against" insert which column 4 line 18, for "cotnrol read control column 9 line .25 for puriew" read purview =0 Signed and sealed this 29th day of May 1962c (SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

